A Lonely Giant: The Sparse Satellite Population of M94 Challenges Galaxy Formation (1807.03779v2)

Abstract: The dwarf satellites of giant' Milky Way (MW)-mass galaxies are our primary probes of low-mass dark matter halos. The number and velocities of the satellite galaxies of the MW and M31 initially puzzled galaxy formation theorists, but are now reproduced well by many models. Yet, are the MW's and M31's satellites representative? Were galaxy formation modelsoverfit'? These questions motivate deep searches for satellite galaxies outside the Local Group. We present a deep survey of the classical' satellites ($M_{\star}$$\geqslant$4$\times$10$^5 M_{\odot}$) of the MW-mass galaxy M94 out to 150 kpc projected distance. We find $only\ two$ satellites, each with $M_{\star}{\sim}10^6 M_{\odot}$, compared with 6-12 such satellites in the four other MW-mass systems with comparable data (MW, M31, M81, M101). Using astandard' prescription for occupying dark matter halos (taken from the fully hydrodynamical EAGLE simulation) with galaxies, we find that such a sparse satellite population occurs in $<$0.2% of MW-mass systems $-$ a $<$1% probability among a sample of five (known systems + M94). In order to produce an M94-like system more frequently we make satellite galaxy formation much more stochastic than is currently predicted by dramatically increasing the slope and scatter of the stellar mass-halo mass (SMHM) relation. Surprisingly, the SMHM relation must be altered even for halos masses up to 10$^{11}M_{\odot} -$ significantly above the mass scales predicted to have increased scatter from current hydrodynamical models. The sparse satellite population of this `lonely giant' thus advocates for an important modification to ideas of how the satellites around MW-mass galaxies form.

Overview of the Sparse Satellite Population of M94

The paper conducted by Smercina et al. investigates the satellite galaxy population of M94, a galaxy with a mass comparable to the Milky Way. Their research raises fundamental questions regarding the representation of satellite galaxies outside the Local Group and the models used in galaxy formation theories. This deep survey of M94, stretching to a projected distance of 150 kpc, identified only two satellite galaxies with stellar masses of approximately $10^6 M_{\odot}$, which stands in stark contrast to the expectations set by other Milky Way-mass galaxies, such as the MW, M31, M81, and M101, each hosting between 6-12 comparable satellites.

The rarity of encountering such a sparse satellite population—occurring in less than 0.2% of MW-mass systems according to a standard model derived from the hydrodynamical EAGLE simulation—suggests potential overfitting in current models of galaxy formation. For an M94-like system to occur more frequently, the paper proposes increasing the slope and scatter of the stellar mass-halo mass (SMHM) relation, even out to halo masses of $10^{11}M_{\odot}$, which is markedly above the mass scales previously thought to exhibit increased stochasticity.

Observations and Completeness

Data obtained from the Hyper Suprime-Cam of the Subaru Observatory were meticulously processed to identify low surface brightness dwarf galaxies within the designated fields. Completeness tests were conducted using artificial galaxies, confirming an 85% detection rate for objects with stellar masses greater than $4\times10^5 M_{\odot}$, and near certainty for detecting more massive galaxies.

Implications for Galaxy Formation Models

The findings challenge the fidelity of the standard halo occupation model when applied universally across MW-mass galaxies. The sparse satellite population of M94 requires a revision of how satellites form around such galaxies, advocating for increased stochasticity in galaxy formation processes and more variability in the SMHM relation than traditionally anticipated.

Conclusion

This paper argues for significant alterations to current galaxy formation models, emphasizing a potentially crucial paradigm shift in understanding satellite formation about MW-mass systems. By presenting provocative findings, it highlights the deficiencies of conventional models and beckons further exploration into the stochastic nature of galaxy formation, especially for satellite systems comparable to that observed around M94. This investigation is a vital contribution to the body of research examining the intricacies and unpredictability of satellite galaxy populations and challenges the notion that the Local Group is an adequate representation of the galactic environment.